The invention relates to a method for treating the surface of a part which consists in bringing at least one activated chemical species, such as an activated chemical species contained in a cold plasma, into contact with a surface of the part.
Methods are known for treating the surface of parts by bringing a surface of the part into contact with at least one activated chemical species contained in a cold plasma which may be generated, for example, by an electric discharge between an anode and a cathode, inside a chamber containing a gas and generally a gaseous mixture at a pressure lower than atmospheric pressure.
Plasma contains electrons and also activated species which themselves comprise ionised species and excited neutral species, that is to say, atoms or molecules, some of the electron shells of which are excited under the effect of the electric discharge. The hardening of steel parts by introducing interstitial elements into a surface layer of the steel may be mentioned as an example of the application of surface treatments that use chemical species activated, for example, by electric discharge. The interstitials normally used for hardening the steel are principally nitrogen, carbon and boron. The treatment consists in generating a plasma, for example by an electric discharge, in a gaseous medium containing the interstitial and in bringing the plasma containing activated species into contact with the surface of the part to be treated. The interstitial in the activated state is strongly reactive with respect to the surface of the part, so much so that it penetrates through the surface of the part. During treatment, the part is brought to a temperature which ensures diffusion of the interstitial in the surface layer of the part, to a depth which depends, in particular, on the temperature and the duration of the treatment.
Hardening treatments or, more generally, treatments aiming to modify the surface properties of parts, in particular steel parts, are carried out in this manner by the introduction and diffusion of interstitials in a surface layer of the part.
Normally, a discharge is produced between the part brought to a cathode potential and an anode which may be constituted, for example, by the wall or a portion of the chamber in which the treatment is carried out. In that case, the cold plasma is produced in situ, in the vicinity of the surface of the part to be treated, by electric discharge inside the gaseous medium filling the treatment chamber. The activated species, for example the ionised species or the excited neutral species, are formed in the vicinity of the surface of the part with which they react in order to ensure the provision of an element of the interstitial type.
Generally, the heating and the maintaining of the temperature of the part in order to ensure the diffusion of the interstitial are achieved by electric discharge. It is also possible to provide supplementary means for heating and for maintaining the temperature.
The plasma may also be generated, inside the chamber, by an electromagnetic wave generator, for example a microwave generator or a radio-frequency generator, those means generally requiring pressures of plasma-producing gaseous medium different from the pressures necessary when an electric discharge is used.
The plasma may also be generated in a plasma generator outside the treatment chamber and then transferred into the chamber containing the part to be treated which is heated and maintained at temperature inside the chamber.
In a case where the interstitial used to carry out the treatment is constituted by nitrogen, the gaseous mixture in which the plasma is formed contains nitrogen or a gaseous derivative of nitrogen, those compounds being generally diluted with hydrogen or a mixture of hydrogen and an inert gas, such as argon, or by any other non-reactive diluent mixture. An example of a gaseous mixture commonly used is the mixture N2+H2.
The plasma produced in such a gaseous mixture generally contains ionised species, such as, for example, N+ and N2+ and also excited neutral species, such as, for example, N, N2, NH and H.
It has generally been observed that excited neutral species exhibit good reactivity with respect to the metal surface subjected to the plasma and work efficiently towards the introduction of interstitials at the surface of the part.
In addition, it has been observed that, in the case of a transferred plasma or “post-discharge” plasma, the transfer time of the plasma into the chamber has to be very short in order to preserve reactive species in the plasma. It has also been observed, in the case of a transferred or “post-discharge” plasma, that it is difficult to obtain homogeneous gaseous flow in industrial charges.
In a case where the plasma is produced by an electric discharge, the electric discharge must be maintained in a state of abnormal luminescent discharge, that is to say, a state preceding a state of arc formation between the cathode and the anode.
In a case where the discharge is produced between the part constituting the cathode and a portion of the treatment chamber constituting the anode, there is a not inconsiderable risk that arcs which generate surface defects on the part to be treated will be formed.
In addition, the fact that the plasma is applied directly to the part may cause differences in heating between different portions of the part or from one part to another when a charge comprising a plurality of parts is treated inside the chamber. If some portions of the parts are overheated, when they are made of stainless steel, it is possible to form locally, in the layer enriched with interstitials, precipitates, for example of nitride, which substantially impair the corrosion resistance of the surface of the part.
The treatment temperature of the part, for example in the case of a hardening treatment by interstitial nitrogen or carbon carried out on parts made of steel and, more particularly, parts made of austenitic stainless steel, must be carefully regulated in order to control precisely the diffusion of the interstitials in the surface layer of the part.
Provided that a specific temperature, which is, for example, of the order of from 460° C. to 480° C., is not exceeded, in the case of an austenitic stainless steel, there is formed in the surface layer of the part a solid solution of carbon and/or nitrogen in the metal matrix of the steel, over a few micrometres up to a few tens of micrometres, this surface layer being extremely hard and resistant to wear and not impairing the corrosion resistance of the part.
At higher temperatures, a layer of a solid carbon and/or nitrogen solution is formed in the metal matrix and has the disadvantage of also comprising precipitates of nitrides and/or of carbides which substantially impair the corrosion resistance of the surface of the part.
It may be difficult to regulate precisely the temperature of a part, in all regions of the part, in particular when the part has large dimensions and/or extends over a great length, in a direction (bars or tubes). It is also difficult to regulate very precisely the temperature of each of the parts of a batch of parts which are being treated simultaneously in the treatment installation.
Furthermore, when a batch comprising numerous parts is being treated, it is necessary to arrange those parts on a support, which may be, for example, a cathode support of the treatment installation, prior to carrying out the treatment. This arrangement of parts requires the provision of means for supporting and positioning the parts on the cathode support in such a manner that the treated surface of the parts is completely exposed to the plasma which is formed in the electric discharge. Moreover, the arrangement of a large number of parts requires delicate handling and a performance time which may be long.
In the case of parts that have a complex shape and that comprise, for example, small cavities, it is difficult to carry out a treatment which is satisfactory in all portions of the parts.
Likewise, it is not possible to treat parts in the stacked state or to treat wound strips, owing to the fact that the surfaces that are not exposed to the gaseous medium in which the plasma is formed are not subjected to the treatment.
The methods for treating the surface of parts by activated species, generally excited neutral species and, to a lesser extent, ionised species, as currently implemented, therefore have some limitations, although such treatments have proved to be extremely effective in numerous applications.